global

A Global Name Registration Facility

This documentation describes the Global module which consists
of the following functionalities:

registration of global names;global locks;maintenance of the fully connected network.

These services are controlled via the process
global_name_server which exists on every node. The global
name server is started automatically when a node is started.
With the term global is meant over a system consisting
of several Erlang nodes.

The ability to globally register names is a central concept in
the programming of distributed Erlang systems. In this module,
the equivalent of the register/2 and whereis/1
BIFs (for local name registration) are implemented, but for a
network of Erlang nodes. A registered name is an alias for a
process identifier (pid). The global name server monitors
globally registered pids. If a process terminates, the name will
also be globally unregistered.

The registered names are stored in replica global name tables on
every node. There is no central storage point. Thus,
the translation of a name to a pid is fast, as it is always done
locally. When any action in taken which results in a change to
the global name table, all tables on other nodes are automatically
updated.

Global locks have lock identities and are set on a specific
resource. For instance, the specified resource could be a pid.
When a global lock is set, access to the locked resource is
denied for all other resources other than the lock requester.

Both the registration and lock functionalities are atomic. All
nodes involved in these actions will have the same view of
the information.

The global name server also performs the critical task of
continuously monitoring changes in node configuration: if a node
which runs a globally registered process goes down, the name
will be globally unregistered. To this end the global name
server subscribes to nodeup and nodedown messages
sent from the net_kernel module. Relevant Kernel
application variables in this context are net_setuptime,
net_ticktime, and dist_auto_connect. See also
kernel(6).

The name server will also maintain a fully connected network. For
example, if node N1 connects to node N2 (which is
already connected to N3), the global name servers on the
nodes N1 and N3 will make sure that also N1
and N3 are connected. If this is not desired, the command
line flag -connect_all false can be used (see also
erl(1)). In
this case the name registration facility cannot be used, but the
lock mechanism will still work.

If the global name server fails to connect nodes (N1 and
N3 in the example above) a warning event is sent to the
error logger. The presence of such an event does not exclude the
possibility that the nodes will later connect--one can for
example try the command rpc:call(N1, net_adm, ping, [N2]) in
the Erlang shell--but it indicates some kind of problem with
the network.

Note!

If the fully connected network is not set up properly, the
first thing to try is to increase the value of
net_setuptime.

id() = {ResourceId :: term(), LockRequesterId :: term()}

del_lock(Id) -> true

del_lock(Id, Nodes) -> true

notify_all_name(Name, Pid1, Pid2) -> none

Name = term()

Pid1 = Pid2 = pid()

This function can be used as a name resolving function for
register_name/3 and re_register_name/3. It
unregisters both pids, and sends the message
{global_name_conflict, Name, OtherPid} to both
processes.

random_exit_name(Name, Pid1, Pid2) -> pid()

Name = term()

Pid1 = Pid2 = pid()

This function can be used as a name resolving function for
register_name/3 and re_register_name/3. It
randomly chooses one of the pids for registration and kills
the other one.

random_notify_name(Name, Pid1, Pid2) -> pid()

Name = term()

Pid1 = Pid2 = pid()

This function can be used as a name resolving function for
register_name/3 and re_register_name/3. It
randomly chooses one of the pids for registration, and sends
the message {global_name_conflict, Name} to the other
pid.

register_name(Name, Pid) -> yes | no

register_name(Name, Pid, Resolve) -> yes | no

{Module, Function}
is currently also allowed for backward compatibility, but its use is
deprecated

Globally associates the name Name with a pid, that is,
Globally notifies all nodes of a new global name in a network
of Erlang nodes.

When new nodes are added to the network, they are informed
of the globally registered names that already exist.
The network is also informed of any global names in newly
connected nodes. If any name clashes are discovered,
the Resolve function is called. Its purpose is to
decide which pid is correct. If the function crashes, or
returns anything other than one of the pids, the name is
unregistered. This function is called once for each name
clash.

Warning!

If you plan to change code without restarting your system,
you must use an external fun (fun Module:Function/Arity)
as the Resolve function; if you use a
local fun you can never replace the code for the module that
the fun belongs to.

There are three pre-defined resolve functions:
random_exit_name/3, random_notify_name/3, and
notify_all_name/3. If no Resolve function is
defined, random_exit_name is used. This means that one
of the two registered processes will be selected as correct
while the other is killed.

This function is completely synchronous. This means that
when this function returns, the name is either registered on
all nodes or none.

The function returns yes if successful, no if
it fails. For example, no is returned if an attempt
is made to register an already registered process or to
register a process with a name that is already in use.

Note!

Releases up to and including OTP R10 did not check if the
process was already registered. As a consequence the
global name table could become inconsistent. The old
(buggy) behavior can be chosen by giving the Kernel
application variable global_multi_name_action the
value allow.

If a process with a registered name dies, or the node goes
down, the name is unregistered on all nodes.

set_lock(Id, Nodes, Retries) -> boolean()

Sets a lock on the specified nodes (or on all nodes if none
are specified) on ResourceId for
LockRequesterId. If a lock already exists on
ResourceId for another requester than
LockRequesterId, and Retries is not equal to 0,
the process sleeps for a while and will try to execute
the action later. When Retries attempts have been made,
false is returned, otherwise true. If
Retries is infinity, true is eventually
returned (unless the lock is never released).

If no value for Retries is given, infinity is
used.

This function is completely synchronous.

If a process which holds a lock dies, or the node goes
down, the locks held by the process are deleted.

The global name server keeps track of all processes sharing
the same lock, that is, if two processes set the same lock,
both processes must delete the lock.

This function does not address the problem of a deadlock. A
deadlock can never occur as long as processes only lock one
resource at a time. But if some processes try to lock two or
more resources, a deadlock may occur. It is up to the
application to detect and rectify a deadlock.

Note!

Some values of ResourceId should be avoided or
Erlang/OTP will not work properly. A list of resources to
avoid: global, dist_ac,
mnesia_table_lock, mnesia_adjust_log_writes,
pg2.

sync() -> ok | {error, Reason :: term()}

Synchronizes the global name server with all nodes known to
this node. These are the nodes which are returned from
erlang:nodes(). When this function returns,
the global name server will receive global information from
all nodes. This function can be called when new nodes are
added to the network.

trans(Id, Fun, Nodes, Retries) -> Res | aborted

Sets a lock on Id (using set_lock/3). If this
succeeds, Fun() is evaluated and the result Res
is returned. Returns aborted if the lock attempt
failed. If Retries is set to infinity,
the transaction will not abort.

infinity is the default setting and will be used if
no value is given for Retries.

unregister_name(Name) -> term()

Name = term()

Removes the globally registered name Name from
the network of Erlang nodes.

whereis_name(Name) -> pid() | undefined

Name = term()

Returns the pid with the globally registered name
Name. Returns undefined if the name is not
globally registered.